With *UndecidableInstances* switchd on it seems to work :-)
Thanks for the help!
Gabor
Em sexta-feira, 31 de julho de 2015, Gabor Greif <[email protected]>
escreveu:
> No. I'll switch that on and report back.
>
> Thanks,
>
> Gabor
>
> Em sexta-feira, 31 de julho de 2015, Simon Peyton Jones <
> [email protected]
> <javascript:_e(%7B%7D,'cvml','[email protected]');>> escreveu:
>
>> you need "liberal coverage checking", so UndecidableInstances. Are you
>> doing that?
>>
>> | -----Original Message-----
>> | From: ghc-devs [mailto:[email protected]] On Behalf Of
>> | Richard Eisenberg
>> | Sent: 31 July 2015 16:10
>> | To: Gabor Greif
>> | Cc: ghc-devs
>> | Subject: Re: Fundep question
>> |
>> | Let's rewrite with explicit kind variables, noting that b is also
>> | poly-kinded:
>> |
>> | class Dep k k2 (a :: k) (b :: k2) | a -> b k2
>> | -- if a determines b, it surely determines k2
>> |
>> | instance Dep k * x y => Dep (Maybe k) * (Just x) (Maybe y)
>> |
>> | Actually, even with the kinds explicit, it still looks valid to me.
>> | Post a bug report?
>> |
>> | Richard
>> |
>> | On Jul 31, 2015, at 9:54 AM, Gabor Greif <[email protected]> wrote:
>> |
>> | > Hi all,
>> | >
>> | > say I want to instantiate
>> | >
>> | > class Dep (a :: k) b | a -> b
>> | >
>> | > as
>> | >
>> | > instance Dep x y => Dep (Just x) (Maybe y)
>> | >
>> | > Is this supposed to work? I get "The coverage condition fails"
>> | errors.
>> | >
>> | > For simple cases like
>> | >
>> | > instance Dep True Bool
>> | >
>> | > etc. it seems to work fine.
>> | >
>> | > Thanks and cheers,
>> | >
>> | > Gabor
>> | > _______________________________________________
>> | > ghc-devs mailing list
>> | > [email protected]
>> | > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>> |
>> | _______________________________________________
>> | ghc-devs mailing list
>> | [email protected]
>> | http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>
>
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